1. Technical Field
The present invention relates to an image forming apparatus that forms an image on a sheet material according to an electrophotographic method, and, more particularly, to an image forming apparatus capable of conveying a sheet material while keeping the loop of the sheet material between a transferring portion and a fixing portion at an appropriate amount.
2. Background Art
In an image forming apparatus using an electrophotographic method, a toner image on a photoconductor is first transferred to a sheet material by means of a transferring portion. Thereafter, the sheet material is allowed to pass through a conveying guide, and is guided to a nip of a fixing device. Herein, there is a case in which the trailing edge of the sheet material has not yet passed through the transferring portion when the leading edge of the sheet material enters the nip of the fixing device.
On the other hand, there is a case in which a difference arises between the sheet material conveying speed in the fixing device and the sheet material conveying speed in the transferring portion because of thermal expansion of a pressure roller provided in the fixing device, an individual difference, or a time-dependent change. In this case, if the sheet material conveying speed in the fixing device exceeds the sheet material conveying speed in the transferring portion, the sheet material carrying a not-yet-fixed toner image will be pulled toward the fixing device between the fixing device and the transferring portion, and, as a result, there is a fear that transfer deviation or slippage will occur.
To prevent the occurrence of this transfer deviation phenomenon, it has been conventionally performed to form a loop (i.e., a slackened state) for a sheet material conveyed between the transferring portion and the fixing device. However, there is a fear that the sheet material will be rumpled the next time if an excessive loop is given to the sheet material. Therefore, it is desired that the sheet material be conveyed while keeping its loop amount between the transferring portion and the fixing device at an appropriate amount.
A technique is known as an approach to respond to this request. This technique is carried out in the following way. In an image forming apparatus including a fixing unit and a transferring unit, drive systems are provided independently of each other for the fixing and transferring units, respectively. The upper limit and the lower limit of a loop amount formed by a sheet of paper at the front of an entrance of the fixing unit are detected. When its upper limit is detected, the speed of a driving portion provided on the side of the fixing unit is increased. When its lower limit is detected, the speed of the driving portion provided on the side of the fixing unit is decreased. Based on the loop amount detected in this way, the conveying speed of the sheet material is controlled. (See Japanese Published Unexamined Patent Application No. H7-234604).
However, in the conventional technique, the conveying speed of the sheet material is controlled based on the upper limit and the lower limit of the loop amount detected as above, and hence excellent results have not yet been achieved from the viewpoint of forming an image without transfer deviation or without rumples. In other words, to form an image without transfer deviation or rumples, there is a need to finely detect a loop amount and to reflect this in a control system for controlling the conveying speed. However, in the conventional technique, it is impossible to finely detect the loop amount at multiple stages including intermediate stages between the upper limit and the lower limit. Therefore, satisfactory results have not yet been achieved from the viewpoint of forming an image without transfer deviation or rumples.